The previous invention, described in U.S. Pat. No. 4,434,649, of a caliper measurement device using an annular air bearing has been shown to have certain limitations when applied to paper webs of varying smoothness or webs which contain cockles.
The problems associated with webs of varying smoothness are not errors of mean thickness per se but are manifested in the inability to build large rolls of paper with uniform cross direction density and diameter. The effective thickness of the paper when stacked or rolled is a function not only of thickness but also the degree to which the surface irregularities nest together. Therefore a roll of rough surfaced paper wound with fixed windup tension will have a greater diameter and appear more dense than a smooth surfaced sheet of the same mean thickness.
Cockled paper has small puckered areas usually caused by uneven drying. These areas cause the small area peak to peak excursions of the sheet surfaces about the plane of the sheet to exceed the thickness of the actual sheet cross section. This introduces an error into the caliper measurement unless the forces acting on the sheet can overcome the forces in the sheet thereby flattening the cockles in the area of measurement.
The annular air bearing caliper described in U.S. Pat. No. 4,434,649 measures the mean thickness of a sheet or web of paper. This will give optimum results when building a reel of paper only when the web is uniformly smooth in the cross machine direction.